Sorry, I think we're having two different conversations. The video on
Dean's site is presented as "an object lesson in Newton's First Law,
the law of inertia." I was commenting on the use of the video and
similar demos in the context of N1L, not any broader discussion of
inertia. If you have a version of that law that doesn't involve
assuming a zero net/unbalanced external force, I'd be interested in
hearing it.
Mike
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Michael Porter
Colonel By Secondary School
Ottawa, Ontario, Canada